Breast cancer exhibits enormous cellular, genomic and biological complexity wherein multiple gene aberrations act to drive tumorigenesis and cancer progression. The long term goal of the proposed studies is to understand the cellular and molecular mechanisms in the regulation of breast cancer development and metastasis. Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase that plays a major role in mediating signal transduction by integrins as well as growth factor receptors in the regulation of cell adhesion, migration, survival, proliferation and differentiation in a variety of cells. FAK has been implicated in human breast cancer as well as other malignancies. However, the cellular and molecular mechanisms by which FAK promotes mammary tumorigenesis in vivo are still not well understood. We showed recently that conditional knockout of FAK in the mammary epithelium suppressed mammary tumorigenesis and progression by affecting MaCSCs in a well characterized breast cancer mouse model. To investigate the unique role of FAK as both a tyrosine kinase and a scaffold in intracellular signaling in breast cancer, we have recently created two novel FAK knock-in mutant mice with the kinase-defective allele (FAK +/KD mice) or the P878/881A mutation allele (FAK +/PA mice) to disrupt its signaling through tyrosine kinase activity or specific scaffolding function for endophilin A2 phosphorylation by Src and regulation of MT1-MMP, respectively. Preliminary analyses of the PA mutant knockin mice using the MMTV-PyMT model (PA/PA-MT mice) revealed that FAK mediated endophilin A2 phosphorylation by Src plays an important role in mammary tumor growth and metastasis by promoting epithelial to mesenchymal transition (EMT) of mammary tumor cells, and by maintaining the content and tumorigenicity of MaCSCs. In addition, we identified a function of FAK in the maintenance of normal MaSCs and found that kinase-independent functions of FAK were able to promote self-renewal of MaSCs. Lastly, we also obtained preliminary results suggesting an important role of FAK in regulation of human MaCSCs, which is consistent with our more extensive findings in mouse models. Based on these preliminary and previous studies, we propose to 1). determine the mechanism of FAK scaffold function in mammary tumor growth and metastasis through regulation of EMT and MaCSCs, 2). examine the role of FAK kinase activity and dissect the downstream kinase-dependent and -independent signaling pathways in breast cancer development and progression, and 3). explore the strategies of targeting FAK scaffold and kinase functions in MaCSCs for breast cancer therapy. Together, these studies will provide significant insights into the molecular and cellular mechanisms of breast cancer that may contribute to novel therapies for this devastating disease.